There are plenty of Googlers, Applers (?), Amazonians or Netflixers posting on HN but not identifying themselves as such. Likewise some FBers do identify themselves as such. I only see it in technical posts (open hardware, folly, oculus etc) because I find the discussion of social networks being good/bad boring.
But definitely if there were a post about why fb is evil and I worked at fb I probably wouldn’t mention that. You don’t see that many google rms defending google on HN either.